Electrical alternans relate to the differences in electrical potential at corresponding points between alternate heartbeats. T-wave alternans or alternation is a regular or beat-to-beat variation of the ST-segment or T-wave of an electrocardiogram (ECG) which repeats itself every beat group (e.g., two beats, three beats or four beats) and has been linked to underlying cardiac electrical instability. Typically, by enumerating all consecutive heart beats of a patient, beats with an odd number are referred to as “odd beats” and beats with an even number are referred to as “even beats.” A patient's odd and even heartbeats may exhibit different electrical properties of diagnostic significance which can be detected from an ECG.
T-wave alternans may be caused by changes in ion exchange during repolarization. If there is an abrupt change in the repolarization period of one beat, the heart attempts to readjust on the following beat. This is manifested as an alternating change in the action potential duration. In the surface ECG this is seen primarily as a change in T-wave. For an implanted medical device such as a cardiac pacemaker, the electrogram (EGM) also shows a change in T-wave. Thus, the term T-wave as used herein may refer to a portion of the ventricular QRS-T-wave complex that includes the T-wave and the QRS-T segment. The alternating feature of T-wave alternans can be detected by examination, for example, of the QT interval, T-wave width, T-wave amplitude and morphology, etc.
T-wave alternans has been demonstrated in many studies as a strong predictor of mortality, independent of left ventricular ejection fraction (LVEF). More specifically, it has become well known that T-wave alternans have predictive value for arrhythmic events such as tachyarrhythmias. Additionally, T-wave alternans has been determined to be an indicator of various forms of disordered ventricular repolarization, including disorders found in patients with cardiomyopathy, mild to moderate heart failure, and congestive heart failure.
A recent publication by Narayan et al., entitled “T-wave alternans phase following ventricular extrasystoles predicts arrhythmia-free survival” (Heart Rhythm 2005, Vol 2, No 3: 234-41), which is incorporated herein by reference, concluded that phase reversal in T-wave alternans following single ventricular extrasystoles predicts spontaneous ventricular arrhythmias and all-cause mortality in patients with moderate ischemic left ventricular (LV) dysfunction and was a better predictor than positive T-wave alternans or programmed ventricular stimulation. However, the Narayan publication did not explain how its findings could be used outside of a clinical setting.